Papers used in tissue and toweling grades that require good absorbency also require a high level of wet strength in order to maintain their structural integrity under the mechanical stresses of removing moisture from skin and other surfaces. Measures needed to satisfy both these requirements tend to conflict.
For instance, the rate of absorption of water into paper is generally reduced by such effective wet-strength resins as acid-curing wet-strength resins like urea-formaldehyde and melamine-formaldehyde resins, and neutral- or alkaline-curing resins like polyaminoamide-epichlorohydrin, polyamine-epichlorohydrin, and other amine polymer-epichlorohydrin resins.
Of the permanent wet-strength resins, the neutral or alkaline-curing resins often produce a softer, more absorbent sheet than do the acid-curing urea-formaldehyde and melamine-formaldehyde resins, but they still reduce the rate of water absorption of the paper significantly.
On the other hand, neutral- or acid-curing resins containing aldehyde groups that have a less adverse effect on the rate of absorption, such as dialdehyde starch and glyoxal-modified acrylamide polymers, impart only temporary wet-strength.
With a permanent wet-strength resin, about 80 to 90 percent of the wet strength measured after 10 seconds soaking will persist after two hours soaking, while with a temporary wet-strength resin, typically only one-third to two-thirds of the "10-second" wet strength will persist after two hours.
It is known to use surface-active agents or debonders, dried into the sheet, to facilitate the penetration of water into the paper when it is wet by its use to wipe or dry the skin, but these agents concurrently weaken the dry strength of the sheet, which lowers the wet strength, because the absolute wet strength of a sheet made of a particular pulp under given conditions with a given amount of wet-strength resin will tend to be lowered in direct proportion to its dry strength.
It is known from U.S. Pat. Nos. 3,058,873, 3,049,469, and 3,998,690, and in the Proceedings of the 1983 TAPPI Papermakers Conference, Portland, Oreg., pp. 191-195, that the neutral or alkaline-curing thermosetting wet-strength resins become more effective in imparting wet strength and increasing dry strength, if they are used in conjunction with a water-soluble carboxyl-bearing polymers, such as carboxymethylcellulose (CMC).
It is also known, for instance from U.S. Pat. No. 3,049,469, to combine a thermosetting cationic wet-strength resin and an anionic polyacrylamide, for improved wet and dry tensile strengths in paper. However, it is also known, for instance from U.S. Pat. Nos. 3,332,834, 3,790,514, 3,660,338, and 3,667,888, that combinations of non-thermosetting cationic polymers with anionic water-soluble polymers, those containing carboxyl groups or carboxylate ions and anionic polymers and copolymers of acrylamide, or poly(acrylic acid) or its salts, will increase the dry strength of paper, while imparting little or no wet strength.
With these combinations, it is also known, for instance from Reynolds, Ch. 6 in "Dry Strength Additives", W. F. Reynolds, ed., TAPPI Press, Atlanta, 1980; FIGS. 6-9, p. 141, that the improvement in dry strength rises to a maximum, then declines as the ratio of anionic polymer to cationic polymer increases.
For use in tissue and toweling, it would be desirable to have a paper that, while maintaining needed dry strength, combines high permanent wet strength with rapid absorption of water.